Fuel composition for diesel engines

ABSTRACT

A fuel composition for diesel engines comprising: (1) up to 80% volume of one or more intermediate fractions selected from the group consisting of a straight-run ‘kerosene fraction, a straight-run light oil fraction, a cracked light oil fraction, a cracked kerosene fraction and hydrodesulphurised products of these; and (2) at least 20% volume of gas-to-liquid product, wherein, in a test of oxidation stability in accordance with the EN14112 test, the time until a change of electrical conductivity occurs at a liquid temperature of 150° C. is at least four hours.

FIELD OF THE INVENTION

The present invention relates to a fuel composition for diesel engines.

BACKGROUND OF THE INVENTION

As a method of reducing the amount of particulates (substances in particle form: hereinbelow abbreviated as “PM”) in exhaust gas, the various manufacturers of diesel engines are studying injection into the cylinder at higher pressure in order to reduce the size of particles in the fuel and to improve the optimisation of penetration. Fuel injection is therefore tending to be performed at higher pressures: for example, whereas, in the case of a conventional fuel pump type injection device, the fuel pressure was 70 to 100 MPa, in a so-called “pressure accumulation” common rail type fuel injection device, the fuel pressure is raised to about 140 to 180 MPa.

In such common rail type fuel injection devices, the temperature of the fuel passing through the return line leaving the fuel injection device becomes high due to pressurisation of the fuel, so there is a possibility that the fuel temperature in the fuel tank, where the fuel is in prolonged contact with air, may reach a temperature of about 150° C., which is in the vicinity of the initial boiling point of commercially marketed light oil. Specifically, although it is known that, in general, fuel oil is more easily oxidised as the temperature is raised, in a system having a fuel injection device as described above, oxidative degradation is promoted by high temperature during circulation of the fuel oil through the fuel system. As a result, there is a risk of problems arising such as a failure to achieve optimum fuel injection due to deposition at the injector nozzle, etc. of a gummy fraction generated in the fuel oil.

Studies are therefore being conducted regarding light oil compositions which are of excellent oxidation stability. However, the typically employed methods of evaluation of oxidation stability are JIS K 2287, according to which the oxidation stability is normally evaluated at 100 to 95° C. (gasoline-oxidation stability test method-induction period method), and ASTM D2274-94 (see JP-A-2006-137920 and JP-A-2004-67899); these methods do not evaluate oxidation stability at temperatures as high as 150° C.

In this description, “JIS” refers to Japanese Industrial Standards.

SUMMARY OF THE INVENTION

A fuel composition for diesel engine is provided, comprising (1) up to 80% volume of one or more intermediate fractions selected from the group consisting of a straight-run kerosene fraction, a straight-run light oil fraction, a cracked light oil fraction, a cracked kerosene fraction and hydrodesulphurised products of these; and (2) at least 20% volume of gas-to-liquid product, wherein, in a test of oxidation stability in accordance with the EN14112 test, the time until a change of electrical conductivity occurs at a liquid temperature of 150° C. is at least four hours.

DETAILED DESCRIPTION OF THE INVENTION

A diesel engine fuel oil having oxidation stability capable of withstanding practical use even under a high-temperature environment in a common rail type fuel injection device that is effective for reducing PM in engine exhaust gas is sought. The present invention relates to a fuel composition for diesel engines of the common rail type, of excellent oxidation stability at high temperature.

It has now been found that GTL (gas-to-liquid product) has extremely high high-temperature oxidation stability and furthermore that by blending GTL with a light oil composition the amount of antioxidant required to prevent oxidation at high temperature can be enormously reduced.

Specifically, in accordance with the present invention there is provided a fuel composition for diesel engines comprising: (1) up to 80% volume of one or more intermediate fractions selected from the group consisting of a straight-run kerosene fraction, a straight-run light oil fraction, a cracked light oil fraction, a cracked kerosene fraction and hydrodesulphurised products of these; and (2) at least 20% volume of gas-to-liquid product, wherein, in a test of oxidation stability in accordance with the EN14112 test, the time until a change of electrical conductivity occurs at a liquid temperature of 150° C. is at least four hours, preferably at least six hours.

The GTL (also called “synthetic light oil” or “n-paraffin/iso-paraffin fraction”) used in the present invention is a light oil base material substantially comprising saturated hydrocarbons and having one or more of the properties shown in Table 1, preferably all of said properties:

TABLE 1 Units Range Density g/cm³ 0.730 to 0.800 10% distillation ° C. at least 140 temperature 90% distillation ° C. 180 to 360 temperature Cetane number at least 60 Sulphur fraction ppmw no more than 1 Saturated % volume at least 99 hydrocarbons Aromatic % volume no more than 1 hydrocarbons

There are no particular restrictions regarding the method of manufacture of the GTL employed in the present invention so long as it satisfies the above properties. However, the GTL may be obtained by performing a Fischer-Tropsch reaction on synthesis gas. Such synthesis gas may be obtained by partial oxidation or steam reforming, etc. of, for example, natural gas or coal, to obtain long chain alkyl hydrocarbon heavy oil, which is then subjected to hydrocracking and distillation to obtain GTL chiefly as a fraction from a boiling point range of 140° C. to 200° C. (GTL kerosene fraction) or chiefly as a fraction (GTL light oil fraction) from a boiling point range of 200° C. to 370° C. Furthermore, these fractions may be employed in the form of a suitable mixture thereof. Fuels produced by way of such a Fischer-Tropsch reaction may be derived not only from natural gas or coal, but also from natural gas liquids, petroleum or shale oil, petroleum or shale oil processing residues or biomass.

Preferably, the fuel composition according to the present invention comprises at least 50% volume of GTL, more preferably at least 70% volume of GTL, most preferably 100% volume of GTL. If the GTL content is less than 20% volume, a large quantity of antioxidant is necessary in order to achieve sufficient oxidation stability at high temperature.

From the point of view of prevention of oxidation, it is preferable to employ chiefly a GTL kerosene fraction having a boiling point range from 150° C. to 200° C.

The intermediate fraction that may be employed in the present invention is one or more fractions selected from the group consisting of a straight-run kerosene fraction, a straight-run light oil fraction, a cracked light oil fraction, a cracked kerosene fraction and hydrodesulphurised products of these. The straight-run kerosene fraction and straight-run light oil fraction are obtained by normal pressure distillation of crude oil. The cracked light oil fraction and cracked kerosene fraction are obtained by contact cracking or thermocracking or hydrocracking, etc. of heavy oil. It should be noted that, with the object of reducing beforehand the sulphur content of the cracked light oil fraction or cracked kerosene fraction, hydrodesulphurisation treatment, such as the indirect desulphurisation method or direct desulphurisation method, may be performed beforehand prior to contact cracking, thermocracking or hydrocracking, etc. of the heavy oil. The light hydrocarbon fraction that is then produced accompanying this desulphurisation reaction may be employed as a cracked light oil fraction or cracked kerosene fraction. Usually, these fractions are employed by blending so as to achieve the desired density and distillation properties.

The composition according to the present invention includes up to 80% volume of an intermediate fraction. If the content of the intermediate fraction exceeds 80% volume, a large amount of antioxidant is required in order to obtain a light oil composition having sufficient oxidation stability. Preferably, the composition according to the present invention contains up to 50% volume of the intermediate fraction. More preferably, the composition according to the present invention contains up to 20% volume of the intermediate fraction. Most preferably, the composition according to the present invention does not contain an intermediate fraction.

Preferably, the fuel composition for diesel engines according to the present invention satisfies the following properties (1) to (5):

(1) 90% distillation curve distillation temperature no more than 360° C.: if a 90% distillation curve distillation temperature of 360° C. is exceeded, there is a risk that the concentration of PM in the exhaust gas from the diesel engine may become high; this is therefore undesirable. From the point of view of lowering the level of PM in the exhaust gas from the diesel engine, preferably this temperature is no more than 350° C., more preferably no more than 330° C. and most preferably no more than 320° C. The “90% distillation curve distillation temperature” that is here referred to means the distillation temperature at a distillation rate of 90% volume on the distillation curve measured by JIS K 2254;

(2) Cetane number at least 45: if the cetane number is lower than 45, there is a risk that engine emission characteristics may be degraded, with degradation of the starting characteristics at low temperature or lowering of output, or degradation of the engine exhaust gas due to, for example, residual unburnt gas. From the point of view of engine ignition characteristics, a cetane number of at least 48 is preferable, and a cetane number of at least 50 is more preferable. The cetane number that is here referred to means the cetane number measured by JIS K 2280;

3) Saturated aliphatic hydrocarbon compound content at least 85% volume: if the saturated aliphatic hydrocarbon compound content is less than 85% volume, there is a risk that NOx and PM in the engine exhaust gas will be increased. In order to further reduce the amount of NOx and PM in the exhaust gas, the saturated aliphatic hydrocarbon compound content is preferably at least 90% volume, more preferably at least 95% volume. The saturated aliphatic hydrocarbon compound content that is here referred to means the degree of saturation measured by the Petroleum Institute Standard JPI-5S-49-97;

(4) Total aromatic hydrocarbon content no more than 15% volume, and aromatic hydrocarbon content having two or more benzene rings no more than 2% volume: if the total aromatic hydrocarbon content is more than 15% volume, there is a risk of increase of NOx and PM in the engine exhaust gas: in particular, if the aromatic hydrocarbon content having two or more benzene rings is more than 2% volume, this has a particularly adverse effect in terms of increase of PM. In order to further reduce the amount of NOx and PM in the exhaust gas, preferably the total aromatic hydrocarbon content is no more than 10% volume, and the aromatic hydrocarbon content having two or more benzene rings no more than 1% volume; more preferably, the total aromatic hydrocarbon content is no more than 5% volume, and the aromatic hydrocarbon content having two or more benzene rings no more than 0.5% volume. The total aromatic hydrocarbon content that is here referred to means the sum of the single ring aromatic, the double ring aromatic and the triple ring aromatic hydrocarbons measured by the Petroleum Institute Standard JPI-5S-49-97; and the aromatic hydrocarbon content having two or more benzene rings means the sum of the double ring aromatic and the triple ring aromatic hydrocarbons measured by the same standard;

(5) Total sulphur content no more than 10 ppmw: if the total sulphur exceeds 10 ppmw, the amount of sulphur oxides and PM discharged in the engine exhaust gas increases, adversely affecting the performance and durability of the catalyst and PM filter of the engine exhaust gas cleaning device due to the sulphur oxides, and increasing the quantities of the atmospheric pollutants represented by nitrogen oxides, carbon monoxide, unburnt hydrocarbons and PM. In addition, there is a risk of corrosion of the engine interior and peripheral equipment. In order to further lower the adverse effects on the engine exhaust gas cleaning device, the total sulphur should preferably be no more than 5 ppmw, more preferably no more than 2 ppmw and most preferably no more than 1 ppmw. The total sulphur that is here referred to means the sulphur obtained by JIS K 2541-2.

The fuel composition according to the present invention may contain an antioxidant. Examples of antioxidants that may be used in the present invention include any known antioxidant that is miscible with the base material (GTL and intermediate fraction) referred to above. Typical antioxidants are phenol-based and amine-based antioxidants. Preferred phenol-based antioxidants that may be mentioned include: 3,5-di-tertiary butyl-4-hydroxytoluene, 2,6-di-tertiary butyl-4-methylphenol, 2,4-dimethyl-6-tertiary butylphenol, 2,6-di-tertiary butylphenol, or mixed tertiary butylphenols; preferred amine-based antioxidants that may be mentioned include phenylene diamine-based antioxidants containing an alkyl group and/or an aryl group, such as N-isopropyl-N′-phenyl-p-phenylene diamine, N-(1,3-dimethyl butyl)-N′-phenyl-p-phenylene diamine, N-(1-methylheptyl)-N′-phenyl-p-phenylene diamine, N-cyclohexyl-N′-phenyl-p-phenylene diamine, N,N′-di-secondary butyl-p-phenylene diamine, N, N′-diisopropyl-p-phenylene diamine, N,N′-diphenyl-p-phenylene diamine, N,N′-ditolyl-p-phenylene diamine, or N-tolyl-N′-xylenyl-p-phenylene diamine. These antioxidants may be employed alone, or as a combination of two or more antioxidants. Commercial antioxidants comprising a mixture of antioxidants may also be employed. If the amount of antioxidant is more than necessary, this may be disadvantageous in that costs are increased and remodelling of the equipment used to perform addition thereof may become necessary. The blending amount of antioxidant is preferably no more than 200 ppm, more preferably no more than 100 ppm; even more preferably the blending amount of antioxidant is no more than 50 ppm, and most preferably no more than 10 ppm. It should be noted that ppm as referred to herein means the weight parts of antioxidant per million volume parts of the liquid base material.

To the fuel oil composition according to the present invention, there may be added if required a low temperature flowability improver, with a view to avoiding, for example, difficulties in shipping due to precipitation of wax constituents at low temperature or blockage of the filter that is installed in the fuel system of the vehicle. As the low temperature flowability improver, any known low-temperature flowability improver may be employed, so long as it is miscible with the base material (GTL and intermediate fraction) described above. Typical low-temperature flowability improvers are commercially available low-temperature flowability improvers such as ethylene-vinyl acetate copolymers, ethylene alkyl acrylate copolymers, alkenyl amide succinates, polyethylene chloride, or polyalkyl acrylate. These compounds may be employed either alone or as a combination of two or more such compounds. Of these, ethylene-vinyl acetate copolymers and alkenyl amide succinates are particularly preferable. As the content of the low temperature flowability improver, for example a suitable amount may be blended such as to satisfy the flowability point and blockage point specified in JIS K 2204, which is the JIS standard for light oil; usually, however, the amount will be 50 to 1000 ppmw. The flowability point that is here referred to means the flowability point obtained by JIS K 2269; and the blockage point means the blockage point obtained by JIS K 2288.

If required, a lubricity improver may be added to the fuel oil composition according to the present invention in order to prevent wear of, for example, fuel supply pump components. Any known lubricity improver may be employed as the lubricity improver so long as it is miscible with the base material (GTL and intermediate fraction) described above. Typical lubricity improvers are commercially available lubricity improvers such as acid-based lubricity improvers, whose chief constituent is a fatty acid, and ester-based lubricity improvers, whose chief constituent is a glycerin mono fatty acid ester. These compounds made be employed alone or in the form of a combination of two or more such compounds. As the fatty acids employed in such lubricity improvers, fatty acids of carbon number 12 to 22, preferably unsaturated fatty acids of carbon number about 18, specifically, whose chief constituents are a mixture of, for example, oleic acid, linolic acid or linolenic acid, are preferred. The lubricity improver may be added so that the wear scar WS1.4 value in an HFRR (high-frequency reciprocating rig) of the fuel oil composition after addition of the lubricity improver is no more than 500 μm, preferably no more than 460 μm: the concentration thereof is usually 50 to 1000 ppmw. The WS 1.4 value in an HFRR that is here referred to means the value obtained in accordance with the Petroleum Institute Standard JPI-5S-50-98.

Any other desired additives may be added to the fuel oil composition according to the present invention, in a range that does not depart from the scope of the present invention. Examples of these additives that may be given include cetane improvers such as alkyl nitrate derivatives or organic peroxides, cleansing agents such as amine salts of alkenyl succinate derivatives, metal deactivators such as salicylidene derivatives, de-icing agents such as polyglycol ether, aliphatic amines, anti-corrosion agents such as alkenyl succinic acid esters, anti-static additives such as anionic, cationic or amphoteric surfactants, or anti-foaming agents such as silicones. These additives may be employed either alone or in the form of a combination of two or more such additives. The addition amount may be suitably selected, but is for example no more than 0.2% weight with respect to the fuel oil composition.

EXAMPLES

The present invention will now be described in detail below with reference to practical examples and comparative examples. However, the present invention is not restricted in any way to these examples.

1. Test Conditions Oxidation Stability Test

A ‘Rancimat 743’ of the Swiss company Metrohm in accordance with the test procedure of the European Standard Test EN 14112 (April 2003), which is an oxidation stability test relating to fatty acid methyl esters employed as biodiesel, was employed as the test equipment. About 3 g of sample is taken into the reaction container of the test equipment, air is blown in at a flow rate of 10 litres per hour into the sample, maintained at 110° C. or 150° C., the discharged gas is passed to a conductivity measuring cell to which 50 mL of distilled water has been added, and the time taken from the start of the test until formation of oxidation products (formic acid and acetic acid may be considered as the chief constituents thereof) abruptly increases is found from the change in conductivity: this is taken as the induction time. Although this EN 14112 test is run at a test temperature of 110° C., in the present test, a test temperature of 150° C. was adopted in order to evaluate oxidation stability at high temperature.

Base Materials

The properties of the GTL and intermediate fraction employed for the base material in the oxidation stability test are shown in Table 2. The method of manufacturing these base materials is as indicated below.

Base material 1 to base material 3 (GTL): GTL base material of the properties shown in Table 2 was obtained using the Shell Middle Distillate Synthesis (SMDS) process, in which hydrocracking and isomerization are performed on a catalyst, after synthesis of waxy straight-chain alkyl hydrocarbons by a Fischer-Tropsch reaction of synthesis gas of carbon monoxide and hydrogen (CO+H₂) by partial oxidation of natural gas.

Base material 4 (intermediate fraction): an intermediate fraction of the properties shown in Table 2 was obtained by hydrogenation treatment under the reaction conditions: reaction pressure 2 to 5 MPa, reaction temperature 250 to 350° C., LHSV 0.5 to 6.0 h⁻¹ hydrogen/oil ratio 50 to 250 Nm³/m³, on a desulphurised catalyst in which cobalt/molybdenum is carried on an alumina carrier, using as raw material a straight run kerosene fraction of boiling point range about 150 to 270° C. obtained by distillation of Middle East crude at normal pressure.

Base material 5 (intermediate fraction): an intermediate fraction of the properties shown in Table 2 was obtained by hydrogenation treatment under the reaction conditions: reaction pressure 4 to 8 MPa, reaction temperature 300 to 400° C., LHSV 0.5 to 2.0 h⁻¹, hydrogen/oil ratio 200 to 350 Nm³/m³, on a desulphurised catalyst in which cobalt/molybdenum is carried on an alumina carrier, using as raw material a product obtained by desulphurising beforehand by the indirect desulphurisation method a reduced pressure light oil fraction of boiling point range about 300 to 550° C. obtained by further reduced pressure distillation of the normal pressure distillation residue oil in respect of a straight run light oil fraction of boiling point range 200 to 370° C. obtained by normal pressure distillation of Middle East crude, in the amount of 80 to 100% volume, and then mixing with 20 to 0% volume of a light contact-cracked light oil fraction (light cycle oil) with a boiling point range of about 200 to 350° C. obtained by contact cracking by the fluid contact-cracking method. This base material 5 has the same composition properties as commercial light oil.

Base material 6 (intermediate fraction): an intermediate fraction of the properties shown in Table 2 was obtained by hydrogenation treatment under the reaction conditions: reaction pressure 4 to 8 MPa, reaction temperature 300 to 400° C., LHSV 0.5 to 2.0 h⁻¹, hydrogen/oil ratio 200 to 350 Nm³/m³, on a desulphurised catalyst in which cobalt/molybdenum is carried on an alumina carrier, using as raw material a product obtained by taking the remaining reduced pressure residue oil from which said reduced pressure light oil fraction has been removed, obtained by further reduced pressure distillation of the normal pressure distillation residue oil in respect of a straight run light oil fraction of boiling point range about 200 to 370° C. obtained by normal pressure distillation of Middle East crude, in the amount of 80 to 100% volume, and then mixing with 20 to 0% volume of a light hot-cracked light oil fraction (light coker gas oil) with a boiling point range of about 200 to 370° C. obtained by hot cracking by the flexicoking method. This base material 6 has the same composition properties as commercial light oil.

TABLE 2 Base material number 1 2 3 4 5 6 Type GTL intermediate fraction Boiling point 149 to 203 199 to 324 209 to 358 144 to 269 143 to 360 178 to 370 range (° C.) 90% fraction 186 312 341 249 338 346 point (° C.) Cetane number 64 78 78 48 54 53 Saturated 100 100 100 83.2 81.5 77.9 aliphatics (% volume) Total — — — 16.8 18.5 22.1 aromatics (% volume) Double ring or — — — 0.3 2.1 2.4 more aromatics (% volume) Sulphur <1 <1 <1 8 5 7 content (ppmw) Density 0.737 0.779 0.785 0.794 0.831 0.841 (15° C., g/cm³)

Antioxidants

As a phenol-based antioxidant, Ionol™ (3,5-di-tertiary butyl-4-hydroxytoluene) manufactured by Shell Chemicals Japan Ltd., was employed, and, as an amine-based antioxidant, NU No. 400™ (containing as active constituent 50% weight of alkyl-aryl phenylene diamine) manufactured by Nikki Universal Co. Ltd, was employed.

2. Practical and Comparative Examples Practical Example 1 and Comparative Example 2 to Comparative Example 7

Oxidation stability tests were conducted using as samples the base materials set out in Table 2. The induction times obtained are shown in Table 3.

TABLE 3 Practical Comp. Comp. Comp. Comp. Comp. Comp. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Base 1   2   3   4   5   6   1:1 material mixture of number 3 and 5 Induction 6 or more 6 or 6 or 6 or 6 or 6 or 6 or more time (110° C.) more more more more more Induction 4.3 0.8 0.9 1.5 1.8 1.6 2.0 time (150° C.)

As shown in Table 3, the various base materials satisfied the standard of six hours or more at 110° C. set out in EN 14112. Below high temperature (150° C.), the induction time of all the base materials is lowered, but, in the case of the GTL kerosene fraction of Practical Example 1, compared with the other base materials, there was an improvement in oxidation stability at high temperature. Also, it was found that the GTL light oil fractions (Comparative Examples 2 and 3) showed inferior oxidation stability compared with the intermediate fraction base materials (Comparative Examples 4 to 6).

Comparative Examples 8 and 9

The induction time was measured using as a sample the product obtained by adding antioxidant to base material 5, which does not contain GTL. When the amounts of Ionol™ (phenol-based antioxidant) and NU No. 400™ (amine-based antioxidant) are respectively 350 ppmw and 250 ppmw (calculated as the active constituents, here and hereinbelow), an induction time of more than six hours was measured at 150° C. The results are shown in Table 4.

TABLE 4 Comp. Comp. Ex. 8 Ex. 9 Base material No. 5 5 Antioxidant phenol-based 350 — (ppmw) amine-based — 250 Induction time (150° C.) 6.5 6.2

Practical Examples 10 to 12

Oxidation stability was evaluated using a sample of Practical Example 10, i.e. the product obtained by adding 1 ppm of NU No. 400™ (amine-based antioxidant) to base material 1 (GTL kerosene fraction). Also, the oxidation stability of samples of Practical Example 11 and Practical Example 12, respectively obtained by adding 75 ppm of Ionol™ (phenol-based antioxidant) and 50 ppmw of NU No. 400™ (amine-based antioxidant) to base material No. 3 (GTL light oil fraction) was evaluated. It was found that the induction time was 6.4 hours in the case of Practical Example 10 (antioxidant 1 ppmw), 6.2 hours in the case of Practical Example 11 (antioxidant 75 ppmw), and 6.1 hours in the case of Practical Example 12 (antioxidant 50 ppmw); as a comparison with Comparative Examples 8 and 9 in which only the intermediate fraction was used as base material, an equivalent oxidation stability could thus be achieved with a much smaller added amount of antioxidant. The results are shown in Table 5.

TABLE 5 Pract. Pract. Pract. Ex. 10 Ex. 11 Ex. 12 Base material No. 1 3 3 Antioxidant phenol-based 75 (ppmw) amine-based 1 — 50 Induction time (150° C.) 6.4 6.2 6.1

As is clear by comparing Practical Examples 11 and 12 (Table 5) and Comparative Examples 8 and 9 (Table 4), even though the GTL base material itself is inferior in oxidation stability compared with the intermediate fraction, when used together with an antioxidant, the GTL base material made it possible to achieve equivalent oxidation stability with an amount of antioxidant that was very much less than in the case of the intermediate fraction. From this fact, it appears that GTL has some kind of synergetic effect on the performance of the antioxidant.

Practical Examples 13 to 20

The oxidation stability of various samples in which 8 to 200 ppmw of antioxidant were added to various types of mixed base material (GTL content 80 to 20% volume), consisting of GTL (base materials 1 and 3) and intermediate fraction (base material 5), was evaluated. The properties of the various samples and the induction times obtained are shown in Table 6. In each case, it was found that equivalent oxidation stability could be achieved with an added amount of antioxidant that was much smaller in comparison with Comparative Examples 8 and 9, in which only the intermediate fraction was used as the base material.

TABLE 6 Pract. Pract. Pract. Pract. Pract. Pract. Pract. Pract. Ex. No. Ex. No. Ex. No. Ex. No. Ex. No. Ex. No. Ex. No. Ex. No. 13 14 15 16 17 18 19 20 Base Base 50 20 20 material material 1 ratio Base 20 30 30 50 70 (% volume) material 3 Base 50 80 material 4 Base 80 80 70 70 50 30 material 5 Antioxidant Phenol- 200 95 based Amine- 8 75 40 180 150 100 based Boiling point range 153-263 152-269 150-363 152-364 163-359 163-359 176-359 192-358 (° C.) 90% fraction point 227 242 333 340 339 339 339 340 (° C.) Cetane number 55 51 55 58 60 60 64 70 Saturated aliphatics 91.6 86.6 85.2 85.2 87.0 87.0 90.8 94.5 (% volume) Total aromatics 8.4 13.4 14.8 14.8 13.0 13.0 9.3 5.6 (% volume) Double ring or more 0.2 0.2 1.7 1.7 1.5 1.5 1.1 0.6 aromatics (% volume) Sulphur (ppmw) 4 6 4 4 4 4 3 2 Induction time 6.5 6.0 6.0 6.1 6.1 6.5 6.3 6.4 (150° C.) 

1. A fuel composition for diesel engines comprising: (1) up to 80% volume of one or more intermediate fractions selected from the group consisting of a straight-run kerosene fraction, a straight-run light oil fraction, a cracked light oil fraction, a cracked kerosene fraction and hydrodesulphurised products of these; and (2) at least 20% volume of gas-to-liquid product, wherein, in a test of oxidation stability in accordance with the EN14112 test, the time until a change of electrical conductivity occurs at a liquid temperature of 150° C. is at least four hours.
 2. The fuel composition of claim 1 having the following properties: (1) 90% distillation curve distillation temperature no more than 360° C.; (2) cetane number at least 45; (3) saturated aliphatic hydrocarbon compound content at least 85% volume; (4) total aromatic hydrocarbon content no more than 15% volume, and aromatic hydrocarbon content having two or more benzene rings no more than 2% volume; and (5) total sulphur content no more than 10 mass ppmw.
 3. The fuel composition of claim 1 comprising no more than 200 ppmw of an antioxidant.
 4. The fuel composition of claim 2 comprising no more than 200 ppmw of an antioxidant.
 5. The fuel composition of claim 3 wherein the antioxidant is a phenol-based antioxidant or an amine-based antioxidant.
 6. The fuel composition of claim 4 wherein the antioxidant is a phenol-based antioxidant or an amine-based antioxidant.
 7. The fuel composition of claim 3 wherein the antioxidant is a phenylene diamine-based antioxidant.
 8. The fuel composition of claim 4 wherein the antioxidant is a phenylene diamine-based antioxidant.
 9. The fuel composition of claim 3 wherein the antioxidant is 3,5-di-tertiary-butyl-4-hydroxytoluene.
 10. The fuel composition of claim 4 wherein the antioxidant is 3,5-di-tertiary-butyl-4-hydroxytoluene.
 11. The fuel composition of claim 3 wherein the time until a change of electrical conductivity occurs at a liquid temperature of 150° C. is at least six hours.
 12. The fuel composition of claim 4 wherein the time until a change of electrical conductivity occurs at a liquid temperature of 150° C. is at least six hours.
 13. The fuel composition of claim 5 wherein the time until a change of electrical conductivity occurs at a liquid temperature of 150° C. is at least six hours.
 14. The fuel composition of claim 6 wherein the time until a change of electrical conductivity occurs at a liquid temperature of 150° C. is at least six hours.
 15. The fuel composition of claim 7 wherein the time until a change of electrical conductivity occurs at a liquid temperature of 150° C. is at least six hours. 